1. Field of the Invention
The invention relates to a vehicular airbag device.
2. Description of the Related Art
In a conventional vehicular airbag device, a variable vent hole is provided so that the vent hole, which is open during the initial stages of airbag deployment, becomes closed by a strap while the airbag expands (see U.S. Pat. No. 7,347,450). The strap is coupled to the lower portion of the deployed airbag on it vehicle rear side and to the vent hole.
In a vehicle equipped with the vehicular airbag device, the airbag deploys between the passenger and an instrument panel when a head-on collision occurs to restrain the upper body of a passenger sitting in a vehicular seat. It should be noted herein that if the passenger sitting in the vehicular seat does not wear a seat belt in a jurisdiction where the use of a seat belt is not legally required, the body of the passenger moves wholly forward with respect to the vehicle due to inertia, and the upper body of the passenger wholly impacts with the airbag. In the structure of the airbag according to the related art, when the strap that is coupled to the lower portion of the airbag on the vehicle rear side is slack, the vent hole opens due to a rise in internal pressure resulting from the impact of the upper body of the passenger when the upper body of the passenger comes into contact with the airbag.
However, if the passenger sitting in the vehicular seat is restrained by the seat belt, the lower portion of the upper body of the passenger is restrained from moving forward with respect to the vehicle, and the direction of impact (the route of impact) of the upper body of the passenger differs from that when the passenger is not restrained by the seat belt. Therefore, the effective stroke of the airbag in restraining the passenger changes, and the reactive force of the airbag necessary to appropriately restrain the passenger changes as well. Further, when the passenger sitting in the vehicular seat is restrained by the seat belt, the lower portion of the upper body of the passenger is restrained from moving forward with respect to the vehicle. In the structure according to the related art, therefore, it is considered that the strap is unlikely to slack and the vent hole does not easily open.